The invention relates to a target detector for detecting targets, provided with transmitter means for simultaneous generation of different electromagnetic waves having different frequencies which are transmitted by means of an emission and detection device, with receiving means for obtaining, by means of the emission and detection device, target signals comprising target information received at different frequencies, and with a signal processing unit for processing the target signals generated by the receiving means to obtain target-representing signals.
Such a target detector is known from DE-B No. 1.223.903. The target detector concerned here is a radar suitable for tracking targets. The different frequencies therefore concern microwaves, while the number of different frequencies is two. The patent specification describes that the application of two radar frequencies lying close together has the advantage of providing two observations which can be compared with each other. Signal processing in this patent specification comprises the comparison of the two received echo signals to determine which signal has the lowest amplitude. Subsequently, the signal having the lowest amplitude is used to obtain target-representing signals. The underlying principle is that the signal having the highest amplitude should comprise a noise signal because a target echo should be present in both signals. The system according to this German patent specification has the disadvantage, that as a result of the system setup, it is limited to the use of two observations (the transmitter pulses with a first and a second frequency). Moreover, the system does not allow room for any individual and absolute assessment of the received signals. As a result of the comparison between both signals, the assessment is relative and subjective. Also the frequencies must be close together, because comparison of the amplitude of the reflected waves resulting from the first and the second transmitter frequency would be pointless. After all, if the frequencies should lie far apart, after reflection from a target different amplitudes appear which are not the result of active jamming by a third party. The amplitude of a reflected wave is dependent on the radar cross-section of a target and the antenna gain, which in turn are a function of the frequency with which the target is illuminated. Because no individual assessment of the reflected signals takes place, the signals, the frequencies of which are very far apart, cannot be compared. This implies that the so-called multipath effect cannot be suppressed because, to allow this, the condition applies that both frequencies should lie relatively far apart (e.g. a factor 4).